1. Field of the Invention
The present invention relates to an anticorrosive treating solution for a semiconductor substrate having an exposed surface of copper or a copper-containing alloy. More particularly, the present invention relates to an anticorrosive treating concentrate which causes no quality deterioration during the carriage or storage.
2. Description of the Related Art
Aluminum, an aluminum alloy, copper and a copper-containing alloy have heretofore been used as various kinds of electrical wirings, for their electrical conductivity.
In recent years, the integration of semiconductor devices has become increasingly high and the wiring pattern formed on the substrate has become increasingly fine. As a result, there are fears of, for example, reduction in circuit response speed caused by finer wiring width and smaller wiring sectional area and reduction in wiring life owing to the activated electromigration (EM) incurred by increased heat generation, current density, etc.
In order to deal with such problems, it has become common to use copper or copper alloy wirings which are superior to aluminum-based wirings in electrical conductivity, heat resistance, electromigration resistance.
As an example, an insulating layer is formed on a semiconductor substrate or the like; in the insulating layer are formed grooves for wiring and via holes for connection with lower layer metal wiring; thereon is formed a metal film made of copper, a copper alloy or the like.
Then, to the surface of the resulting semiconductor substrate is applied CMP (chemical mechanical polishing) to remove the metal film at the portion other than the grooves and the via holes, whereby wirings and plugs are formed. At this time, copper or a copper alloy is exposed at the surface of the resulting semiconductor substrate.
The semiconductor substrate after CMP is ordinarily subjected to surface cleaning with a chemical solution using a cleaning machine, to remove the abrasive and polishing residue sticking to the substrate surface; thereby, a semiconductor substrate having a cleaned surface is obtained.
Then, cleaning with chemical solution and drying are conducted; thereafter, an insulating film (or a protective film) are formed on the wirings, or the substrate after the above cleaning and drying is allowed to stand in the atmosphere before formation of the insulating film. Here, the chemical solution used in cleaning or storage is referred to as anticorrosive treating solution.
Copper is a metal which is easily oxidized or corroded. Therefore, the fresh metal surface after CMP is very active and, during the above-mentioned cleaning or temporary standing in the atmosphere, tends to be oxidized or stained by the oxygen or ions dissolved in the cleaning solution or the like or by the oxygen in the atmosphere or the like.
As mentioned above, when there is formed a pattern of wirings of copper or a copper alloy on a semiconductor substrate by a damascene process or the like, the fresh metal (e.g. copper) surface after CMP is subjected to, for example, a cleaning treatment or subsequent temporary standing in the atmosphere and therefore inevitably undergoes oxidation, etc.
Wirings of copper or its alloy are used in place of conventional wirings of aluminum or its alloy for various excellent properties; however, they undergo oxidation, corrosion, etc. very easily.
The fresh metal surfaces of wirings after CMP is flat and makes easy formation of an insulating film (or a protective film) thereon; however, the fresh metal surfaces are very active and is very easily oxidized or corroded during the above-mentioned cleaning treatment or standing in the atmosphere. This oxidation or corrosion is striking particularly in the portions of the surfaces where grain boundaries are clearly seen.
It has heretofore been investigated to use, for rust-preventive treatment or cleaning treatment of a metal surface, a rust-preventive agent such as benzotriazole or the like. In, for example, JP-A-5-315331 (hereinafter referred to as “conventional case 1”), it is described to use, in cleaning treatment of copper or its alloy provided as a wiring pattern or an electrode in a semiconductor device substrate, an aqueous cleaning solution containing 0.04 to 1% of benzotriazole (BTA), to give rise to a copper/BTA reaction on the copper wiring to form an insoluble copper/BTA film (a protective film) and suppress the corrosion of the copper wiring.
In general, an anticorrosive treating solution is used in the form of an anticorrosive treating concentrate or a water-diluted solution thereof, depending upon the BTA concentration required when the concentrate or the water-diluted solution thereof is used.
In the anticorrosive treating concentrate, the concentration of the effective ingredient contained therein is desired to be as low as possible in view of the carriage cost. For example, when a 0.001% aqueous BTA solution is used in an amount of 1,000 m3 in a cleaning step and when a concentrate therefor is carried in a BTA concentration of 0.001%, the concentrate need be carried in an amount of 1,000 m3; however, when the concentrate is carried in a BTA concentration of 1%, the amount of the concentrate carried is as small as 1 m3, enabling significant reduction in carriage cost to 1/1,000. Thus, the BTA concentration in the anticorrosive treating concentrate is desirably as high as possible.
Meanwhile, benzotriazole type compounds and derivatives thereof have low solubilities in water, in general. In the case of, for example, BTA, its solubility in water is ordinarily only about 1.9 wt-% (hereinafter, % used for concentration refers to wt-%) in water of 20° C.
Therefore, when a 1.9% aqueous BTA solution is carried or stored, the BTA solution becomes a supersaturated aqueous solution when the environmental temperature gets lower than 20° C. When an external impact is applied to the supersaturated aqueous solution or its temperature becomes even lower, the solution is unable to exist as a stable solution and precipitates crystals.
In FIG. 1 is shown the solubility of BTA in water of 0 to 80° C. The solubility of BTA in water is, for example, about 8% at 80° C., about 4% at 30° C., about 1.9% at 20° C. and about 1% at 10° C.
As is clear from FIG. 1, when a 1.9% aqueous BTA solution is cooled to 10° C., the BTA solution first becomes an unstable supersaturated solution and then precipitates about 50% of the BTA originally dissolved in the solution; as a result, the BTA concentration in the solution becomes about 1%. Consequently, there occurs a reduction in BTA concentration, BTA crystals are precipitated in the solution, and the original anticorrosive treating concentrate becomes a concentrate of lower corrosion inhibitability.
In using an aqueous BTA solution in a process for semiconductor device production, this anticorrosive treating solution is ordinarily used at such an effective ingredient concentration as to cause no precipitation of crystals or the like and, moreover, the temperature of the process is controlled so that the original solubility of BTA in water is maintained; therefore, there is substantially no precipitation of BTA (see the column 6, lines 2 to 12 of the conventional case 1).
Meanwhile, anticorrosive treating concentrates such as aqueous BTA solution and the like are exposed to various temperature conditions.
There is considered, for example, a case that a 3% aqueous BTA solution (as an anticorrosive treating concentrate) is produced at an atmospheric temperature of 30° C. by stirring BTA in water for several hours. If this anticorrosive treating concentrate is carried to a cold district or is stored in a store house of cold district at an atmospheric temperature of , for example, 5° C., BTA is precipitated by an amount exceeding the solubility of 5° C. When the concentrate is carried in a container, the precipitated BTA particles form a dense and hard BTA layer on the inner wall, bottom, etc. of the container.
The BTA precipitate, once generated, is difficult to redissolve even by elevating the temperature of the concentrate to 30° C., and redissolution needs restirring of several hours using a stirrer. This strikingly impairs the operability at the operation site and reduces productivity. When the BTA-precipitated anticorrosive treating concentrate is diluted at a predetermined dilution ratio, the diluted solution has a BTA concentration lower than intended and is unable to exhibit the intended performance.
When an anticorrosive treating solution containing a precipitate of fine BTA crystals (hereinafter referred to as BTA particles) is used for cleaning of the surface of a semiconductor substrate, the BTA particles stick onto the surface and stains the surface. As a result, the resist pattern becomes discontinuous, inviting breakage of wirings. Or, the BTA particles stick onto the inner wall of each via hole, increasing the contact resistance. Thus, the resulting semiconductor device has increased defects.
An object of the present invention is to provide an anticorrosive treating concentrate used for corrosion inhibition treatment of a semiconductor substrate having an exposed surface of a metal such as copper, its alloy or the like, wherein the BTA contained as an effective ingredient is not precipitated at the temperatures encountered during the storage or carriage of the concentrate, even when the BTA is contained in a high concentration.
Other object of the present invention is to provide an anticorrosive treating concentrate which, when used for corrosion inhibition treatment of a semiconductor substrate by itself or in a diluted form, can show the same corrosion inhibitability as conventional aqueous BTA solutions do.